The Skaal Amulet is a unique piece of jewelry found in The Elder Scrolls V: Dragonborn. It is worn by Frea and protects her from Miraak's influence.
Location
The only way to obtain this item without the use of the console command (player.additem xx039d4a) is with a reverse pickpocket. After completing the main quest line of The Elder Scrolls V: Dragonborn, Frea becomes available as a follower. Give her another amulet and make sure that she equips it. Then send her home and pickpocket the amulet from her. However, this may not always work.